Liquid crystal device

ABSTRACT

The liquid crystal device comprises a liquid crystal layer, a thin photoconductor layer, and a thin insulator layer sandwiched between the transparent supporting covers having a transparent electrically-conductive layer thereon. The photoconductor layer is cadmium sulphide (CdS) and the insulator layer is zinc sulphide (ZnS).

D w 3 2 S R United State: [111 3,732,429

Braunstein et al. 1 May 8, 1973 541 LIQUID CRYSTAL DEVICE 3,592,527 71971 Conners ..35()/l LC Inventors: Morris Braunstein, Marina del Rey;gi z William P. Bleha J Santa Monica both of Cahf' PrimaryExaminer.lames W. Lawrence [73] Assignee: Hughes Aircraft Company,Culver Assistant ExaminerT. N. Grigsby City, Calif. Att0mey-W. H.MacAllister, Jr.

[22] Filed: Nov. 12, 1971 ABSTRACT [21] Appl' l9828l The liquid crystaldevice comprises a liquid crystal layer, a thin photoconductor layer,and a thin insula- [52] U.S. Cl....250/213 R, 250/833 HP, 350/ LC torlayer sandwiched between the transparent support- [51] [BL C GOZf j31/50 ing covers having a transparent electrically-conductive [58] Fieldof Search ..250/2l3 R, 83.3 HP; layer thereon. The photoconductor layeris cadmium 35 LC sulphide (CdS) and the insulator layer is zinc sulphide(ZnS). [56] References Cited 18 Claims, 8 Drawing Figures UNITED STATESPATENTS 2,892,380 6/1959 Baumann et al. ..350/l60 R unw se/213 LIQUIDCRYSTAL DEVICE BACKGROUND This invention is directed to a liquid crystaldevice which includes a photoconductor layer and insulator layer whichoperate in conjunction with the liquid crystal.

At the present time, numerous nematic liquid crystals, suited for use inaccordance with the teachings of the present invention, are known whichpossess the property of being responsive in localized regions tosuitable applied electric fields by forming regions or areas which areusable as images. For the purpose of better defining the presentinvention it may be noted that liquid crystals can be classified asbeing either nematic, smectic, or cholesteric and that the presentinvention primarily pertains to the use of nematic liquid crystals.Nematic liquid crystals have special properties which afford certainadvantages put to use in the present invention. The present inventiontherefore is concerned broadly with applications for nematic liquidcrystals heretofore unknown notwithstanding extensive work done in thepast with liquid crystals including nematic liquid crystals.

Specific nematic liquid crystals possess the attribute of having liquidcrystalline structure over a wide temperature range (known as thenematic range) wherein such crystals can be affected by electric fieldsof sufficient strength in a manner productive of image formation insidethe crystal. The mechanism of image formation best understood has beencalled scattering of nematic liquid crystal molecules. This involveschange in the optical reflectivity or transmissivity of the crystal.Image formation can, as stated earlier, be produced by using an electricfield. It is not necessarily the case that image formation will occurdue to scatterin g. Other electric field responsive effects presentlyunder study may also underlie the establishment of image defining areasor states. The present inventionin a fundamental sense makes use of anelectric field for image formation inside nematic liquid crystalsirrespective of the precise manner in which the field causes the imagedefining areas and is not necessarily limited to the usage of suchcrystals in a dynamic scattering mode of operation.

In accordance with the present invention various nematic liquid crystalshave been utilized either alone or in combination with other liquidcrystals or other material to provide a nematic liquid crystalsubstance. Different categories of nematic liquid crystal substances areutilized according to the invention to obtain certain desiredproperties, or modifications of nematic liquid crystal properties. Aswill be specified.

hereinafter the nematic liquid crystal substances found best suited forthe purposes of the present invention fall generally into threedifferent categories. These categories may be denoted by reference tothe appearance of a thin film or layer of the various nematic liquidcrystal substances employed and the effect on the appearance of an imageproducing electric field. One category involves substances including orcomposed of a nematic liquid crystal which provide an essentiallytransparent (colorless or clear) layer which can form, by dynamicscattering mode operation, a light scattering electric field inducedimage (as for example milky white) in the substance selected for use.Another category involves substances like those in the first categorywhich include in addition to a nematic liquid crystal a small amount ofa cholesteric material to provide an essentially transparent layer whichcan form, by an emulsion storage effect or dynamic scattering mode ofoperation, a light scattering electric field induced image (as forexample milky white) in the substance. Another category involvessubstances like those in the first category which include in addition toa nematic liquid crystal a pleochroic or dichroic dye material whichimparts the color of the dye to the substance which can be modified,either with or without dynamic scattering mode operation, by an electricfield to produce an electric field induced image of a different shade ofthe color of the dye. Nematic liquid crystal substances fallinggenerally into these categories are given below by way of example andthese substances maybe used in accordance with the teachings of thepresent invention.

SUMMARY In order to aid in the understanding of this invention it can bestated in essentially summary form that it is directed to a liquidcrystal device and method of manufacture thereof. The liquid crystaldevice includes an insulating layer and photoconductive layer in opticalassociation with a nematic or nematic/cholesteric liquid crystal. Inaddition, the device includes substantially transparent electricallyconductive layers by which an electric field can be applied to theliquid crystal.

Accordingly, it is an object of this invention to 'provide a liquidcrystal device which employs a photoconductor in cooperation with ascattering mode of liquid crystal. It is a further object of thisinvention to employ insulator and photoconductor layers, which cooperatewith a liquid crystal for image production in cooperation with theliquid crystal. It is also an object of this invention to provide amethod for the deposition of layers of photoconductors which will beadjacent the liquid Other objects and advantages of this invention willi become apparent from a study of the following portion of thespecification, the claims and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric view of aportion of the basic cell structure and showing a vertical sectiontherethrough together with schematic electrical connections thereto;

FIG. 2 shows a cell similar to the cell of FIG. 1, and shows itsschematic electrical connection;

FIG. 3 shows the manner in which either of the cells of FIG. 1 or 2 canbe employed in conjunction with a projection light source wherein theimage information on the liquid crystal cell is projected onto screen;

FIG. 4 is similar to FIG. 3, but showing a different manner ofapplication of the image information containing exposure light source;

FIG. 5 is a schematic showing of the manner in which either of the cellsof FIGS. 1 and 2 can be employed with a cathode ray tube used for imageinformation, together with reflective display;

FIG. 6 is similar to FIG. 5, showing another schematic arrangementthereof;

FIG. 7 is a schematic drawing of the CdS thin film vacuum depositionchamber;

FIG. 8 is a schematic drawing of post deposition thermal processingfurnace and processing gas controls.

DESCRIPTION FIG. 1 illustrates a first embodiment 10 of a basic cellarrangement. The cell is a laminar structure. Glass substance 12 isordinary clear glass, and since it can be used for transmitted lighttherethrough, it is preferably clear and flat, with its outer surfacessubstantially parallel. Transparent, electrically conductive coating 14is applied to one side of the glass. This coating is specificallyantimony doped tin oxide, commercially known as NESA glass, or tin dopedindium oxide. Glass substance 12 is conveniently one-eighth inch thickwhile the electrically conductive coating 14 therein is of suchthickness as to give a resistance in the range from 1 to 10 ohms persquare. Instead of employing NESA glass, another convenientsubstantially transparent and electrically conductive coating as taughtby Alfred F. Kaspaul US. Pat. No. 3,698,946, can be employed.

Vacuum deposited onto the coating 14 is a thin layer as the insulatorzinc sulphide, illustrated of the layer 16. Thin insulator films ofmaterials such as silicon dioxide or silicon monoxide could also beused. The insulator 16 is in the range from 10 to 10 nm in thickness,and the presently considered optimum thickness is 50 nanometers. Overthe zinc sulphide I layer 16 is vacuum deposited layer 18 of thephotoconductor cadmium sulphide. Photoconductor films of cadmiumselenide could also be used. The photoconductor layer 18 is vacuumdeposited to a thickness of from 2.0 to 12.5 microns.

On top of the photoconductor layer 18 is deposited a liquid crystallayer 20. The liquid crystal layer 20 is a nematic-cholesteric liquidcrystal, of the type defined in an article entitled A New Electric FieldControlled Reflective Optical Storage Effect In Mixed Liquid Crystalssystems by G. H. Heilmeier and Joel E. Goldmacher appearing in Volume 13No. 4 in the Aug. 15, 1968 issue of Applied Physics Letters on pages 132and 133. The thickness of the liquid crystal layer is typically 12.5microns, and as in conventional liquid crystal practice can convenientlyrange from 4 to 25 microns. The thickness is contained by spacerspositioned around the edges of the cell 10, to maintain the volume inwhich the liquid crystal 20 is positioned.

The cell is completed by glass 22 on which is coated transparentconductive layer 24. This structurally correspond to the glass 12 intransparent conductive layer 14, described above. The two glass layers12 and 22 thus define the outer surfaces of cell 10.

Electric contacts 26 and 28 are respectively connected to theelectrically conductive substantially transparent layers 14 and 24.Source 30 of direct current, together with its series control switch 32,are connected across the contacts 26 and 28. The closure of switch 32applies a dc field across the layers between the electrically conductivelayers 14 and 24. AC source 34 and its serially connected switch 36 arealso connected across contacts 26 and 28, in parallel to the dc sourceand switch. The ac source provides ac current from 1,000 to 100,000Hertz.

FIG. 2 illustrates a cell 40 which is a second embodiment of the liquidcrystal device in accordance with this invention. Cell 40 is built offof a plurality of layers, comprising glass layer 42, transparentconductive layer 44, zinc sulphide insulating layer 46, cadmium sulphidephotoconductive layer 48, liquid crystal layer 50, transparentconductive layer 52 and glass cover 54. The layers 42, 44, 46, 48, 52and 54 are respectively of an equal thickness to layers 12, 14, 16, 18,24 and 22 of the cell 10 shown in FIG. 1 and described with respectthereto. Furthermore, the method of formation of these various layers isthe same in both cells. The principle difference between the cells isthat the layer 50 is a nematic liquid crystal, such as those describedin the background references.

Electric contacts 56 and 58 are respectively connected to substantiallytransparent conductive layers 44 and 52. Electrically connected to thesecontacts are the series combination of dc voltage source 60 and itsswitch 62. This is identical to the DC. source 30. In addition, in theembodiment of FIG. 2, a combined dc and ac source is connected acrossthe contacts 56 and 58, in parallel to the dc sources 60. Thus, dcsource 68 is connected to one of the contacts, and through ac source 70and switch 72 to the other of the contacts. By closure of switch 72, anac field with a superimposed dc field is applied between thesubstantially transparent layers 44 and 52. Thus, the cells 10 and 40are the same, except for the differences between the liquid crystallayers, and the fact that the nematic liquid crystal layer 50 also hasthe capability of having a combined field applied thereacross.

' The method of manufacture of the insulating layer 16 and thephotoconductive layer 18, and their corresponding layers 46 and 48, isas follows.

In the first step of manufacture, the transparent conductivecoating-glass combination l2 and 14 is cleaned for use as a substrate ina high vacuum deposition process. There are many methods, well-known inthe art, to accomplish this. In the second step the zinc sulphide filmis vacuum deposited on the conductive coated glass substrates in a highvacuum coating station. The deposition of zinc sulphide is alsowell-known in the art, and the procedure described here is not unique tothe performance of the device. The method used is to evaporateelectronic grade powder or high purity crystalline pieces of zincsulphide from a resistance-heated molybdenum boat or by electronbombardment from an electron-beam evaporation unit. The pressure in thebell jar during deposition is between I l0'and 1X10 Torr is notcritical. The substrates are placed 10 inches above the evaporationsource along with a thickness monitoring instrument, such as quartzcrystal deposition thickness transducer. After glow discharge cleaningof the substrates, the ZnS evaporation commences. The films, typically50 nm.

thick, are deposited in 1 hour, although times of 10 minutes to 2 hourshave been used. The films have also been varied in thickness from 10 to100 nm. with proportional changes in deposition time. After depositionthe films are transparent (absorbing no light in the visible spectrum)and thus are characteristic of nearstoichiometric zinc sulphide.

The third step in the fabrication is the vacuum deposition of the CdSfilm 18 on top of the zinc sulfide layer 16. The deposition is done inthe belljar ofa conventional high vacuum system in the pressure range oflXlto 1X10 Torr. The pressure, however, does not appear to be critical.A cross-section drawing of the instrumentation is shown in FIG. 7. Thevacuum is enclosed by a baseplate 990 and a glass bell jar 910. Thestructures 880 with the zinc sulphide film are held by a stainless steelsubstrate holder 911 and heated by quartz lamps 913. A removable shutter914 shields the substrates until deposition on them is to be commenced.The thickness of the CdS films is directly and continuously monitored ona substrate by the use of optical interference. This is accomplishedwith the use of a laser 915 and detector 916 positioned outside the belljar 910.

The electronic grade CdS powder, in the form of a pressed cylindricalpellet 917, is evaporated from a formed tantalum boat 918 which'is.resistively heated by current passing through buss bars 919 and currentfeedthroughs 920. The boat 918 is designed such that as the CdSevaporates, the pressed pellet 917 settles down into the boat. Thisgives an efficient thermal evaporation over the long period of timerequired for the deposition of the CdS films. While this particularconfiguration gives the characteristics desired, other forms of CdS,such as crystals, and other thermally efficient evaporation sources,such as baffle boxes, could be used to get the same results. Theevaporation rate is controlled by controlling the current to thetantalum boat. The current is set so that 2.5 microns of CdS asmonitored by optical interference is deposited on the structures 880 in1 hour. Typical thicknesses of CdS films are 12.5 microns so thatdeposition times of 2-5 hours are required. Successful results have beenobtained withthicknesses from 2-15 microns and evaporation rates from0.5 to micron/hr.

To avoid the heating of the various elements in the deposition chamberby radiation from the tantalum boat 918 a water cooled plate 921 ispositioned beneath the boat 918 and extending to the diameter of acylindrical stainless steel deposition chamber 922 positioned around theboat. The water-cooling is also used to maintain the temperature of thewall of 7 achieving this can be determined by one skilled in the art.Also the temperatures given can be changed to vary the conductivity andcurrent-voltage characteristics of the CdS films. A range of substratetemperatures from 100 to 200 C and chamber wall temperatures from 40 toC have been used to make CdS films of the given characteristics.

The fourth step in device fabrication is the postdeposition thermalprocessing of the structure 880 as it emerges from step three. Thepreferred process under the present invention can beunderstood withreference to FIG. 8. A controllable furnace 626 has a quartz processingtube 627 of suitable diameter. A gas inlet tube 628 introduces gas whichis preheated by passing through the core of the furnace. The gas exitsthrough short exit tube 630. The temperature (for monitoring andcontrol) near the center of the tube and also near the center of the hotzone is sensed by a thermocouple 631 sheathed in a quartz tube 632. Thestructures 880 are placed in the tube near the center of the hot zone. Acontrollable flow of a single gas or a mixture of gases is provided byvalve 636, flowmeter 635, regulator 634 for cylinder 633 of H S; byvalve 637, flowmeter 642, regulator 643 for cylinder 648 of HCl; byvalve 638, flowmeter 641, regulator 644 for cylinder 647 of O and byvalve 639, flowmeter 640, regulator 645 for cylinder 646 of N It shouldbe recognized that other configurations, obvious to those skilled in theart, can be used. In operation the following procedure is followed.First the structures 880 are inserted in the processing tube 627 and thetube is flushed out using only the N gas from cylinder 646. The furnaceis then brought to 300 C, as indicated by thermocouple 631, with Nflowing at a rate 0.1 to 20 CFH (STP). Nitrogen is used only as a purgemedium. At this point the N is stopped and the processing gas or gasmixture is introduced into the tube. The percentage by volume of thegases that have been used for processing are summarized below:

H 5: percent HCl 0.1 2 percent N 0.1 5 percent 0 10.1 0.5 percent v Thetotal flow rate is from 0.1 to 20 CFl-I (STP).- It should be noted thatthe main constituent gas is H 5 and that pure H 8 can be used as theprocessing gas.

The furnace is brought to the desired temperature, typically 500 C, andkept at that temperature for the desired time, typically 30 minutes.Temperatures from 385 to 550 C and times from 1 minute to 60 minuteshave been successfully used. The particular time and temperature useddepends on the thickness of the CdS film 18 or 48, the substrate 12 or42 material, and the mixture of gases used. Also the devicecharacteristics,

for a given thickness of CdS film, substrate material,

and gas, can be altered by changing the temperature and time. After thedesired time has elapsed the outside of the quartz tube 627 is exposedto the ambient and allowed to cool in approximately 10 minutes to 300 C,as measured by thermocouple 632, at which point the processing gas orgas mixture is turned off and N only is allowed to flow through thetube. When thermocouple 632 indicats 70 C, the structures 880 areremoved from the tube and exposed to the ambient.

While this rapid cooling, caused by exposing the outside of the quartztube 627 to the ambient, produces superior results, devices exhibitingthe desired characteristics can also be obtained by allowing the quartztube 627 to remain completely in the furnace 626 and turning off thepower to the furnace Under these circumstances, the structures 880 cooldown at a rate which is slower by a factor of about 10.

The fifth and final step in the device fabrication is the placing of theliquid crystal layer or 50 and counter electrode-glass substratecombination 22 and 24 or 52 and 54 on the subassembly completed in stepfour. To define the area and thickness of the liquid crystal layer aspacer with an open area in the center, typically made of the insulatorMylar, is placed on the subassembly. The thickness of the spacer istypically 12.5 microns but can be from 6 microns to 25 microns. Theliquid crystal is then spread in the open area in the center of thespacer. An excess of the viscous liquid crystal remains until thecounter-electrode glass assembly is placed down on the liquid crystaland spacer thereby squeezing the excess liquid crystal out of thecenter, and maintaining a layer of liquid with no voids. It should berecognized that this procedure is not unique and that other methodsknown to one skilled in the art could be used to form and define theliquid layer. A supporting clamp is then placed around the device andwires are bonded to the electrodes for electrical contact. The applieddc voltage for operation with either liquid crystal layer 20 or 50 iswith the liquid crystal biased negatively with respect to thephotoconductor.

The double layer structure consisting of the CdS photoconductor and ZnSinsulator allows a higher dark impedance to be obtained in conjunctionwith the liquid crystal layer than the use of just the CdS film. Thehigher dark impedance is obtained without significantly altering thelight-to-dark impedance ratio. As a' result higher contrast betweenscattering and non-scattering areas in the liquid crystal layer isobtained .because of the better impedance match to liquid crystalimpedance levels which give efficient scattering. It can be hypothesizedthat the reason for the improvement of the performance of thephotoconductor with the insulating film is due to the improvednucleation of the CdS film on the ZnS film which results in fewerdefects through the CdS film. It is also possible that a heterojunctionor blocking contact is formed.

It is expected that the photoconductor-insulator structure could be usedto improve the performance of photoconductor films of cadmium selenidewhich has a lower intrinsic resistivity than cadmium sulphide.

In the use, a light having an intensity pattern in accordance with thedesired image is projected onto the cell. This will be called theimaging light, in particular sources and arrangements of imaging will bedescribed hereinafter. After imaging is completed, the liquid crystalstored image can be viewed. This viewing can be accomplished either by aprojected display light, or by reflective observation of the image byfrontal illumination. In either case, a display light is also utilized.

Considering the structure of FIG. 1, both switches 32 v and 36 are open.Thereupon, the imaging light is turned on. The imaging light projectsthe desired image into the cell, both through the nematic-cholestericmixture liquid crystal and the photoconductive layers. With the imaginglight on, the switch 32 is closed for a short time, for example from 0.1to 1 second. When the voltage, typically from 20 to 150 volts, isapplied, the direct current potential field is divided across the liquidcrystal layer 20, and the insulator layer 16 and photoconductive layer18. The imaging light is of such wavelength where the photoconductor issensitive. For cadmium sulphide, the imaging light is thus between 400and 520 nanometers-Where there is no imaging light, the photoconductorand insulator layers totally comprise a higher impedance than the liquidcrystal. Where the imaging light is projected onto the cell, theresistance of the photoconductor decreases, so that it is less than theliquid crystal. Thus, in the exposed area of the photoconductor, thecurrent is caused to flow to induce change in the scattering propertiesof the liquid crystal. In the regions where there was no imaging light,the voltage pulse caused by closure of switch 32, the voltage divisionof such that the principle voltage drop was across the resistor and thephotoconductor and the liquid crystal adjacent thereto is uneffective.Thus, this area remains clear. After switch 32 is open, and the imaginglight removed, the image is retained in the liquid crystal, because ofthe scattering storage properties of the nematic-cholesteric mixture.

At a later time, when it is desired to remove the image stored in theliquid crystal, the liquid crystal layer can be brought to itsnon-scattering state in all regions by closing the switch 36 andapplying the ac field across the liquid crystal, from source 34. Afrequency greater than about 1,000 cycles per second is required toeffect erasure and return the liquid crystal to the clear state. Closureof the switch 36 for 0.5 to 2 seconds is adequate to obtain thiserasure.

In the structure of FIG. 2, the liquid crystal 50 is a nematic material.There is no optical memory, in the sense of scattering condition of sucha material, in the absence of a continuously applied field. Theemployment of the dc field provided by dc source 60 and closure of itsswitch 62 is alternative to the application of the dc field superimposedupon the ac field, by closure of switch 72. The dc field provided bysource 60 is from 10 to 100 volts. On the other hand, the combined fieldcomprises a dc potential from source 68 of from 10 to I00 volts,together with an ac potential from source 70 of from 10 to volts rms ata frequency in the range of from 1,000 to 20,000 cycles per second. Witheither of the switches 62 or 72 closed, the imaging light is turned on.The imaging light is of such wavelength that it affects thephotoconductor. In the case of the cadmium sulphide, the imaging lighthas a wavelength from 400 to 520 nanometers. The illumination light,either for frontal or projective viewing of the images of the liquidcrystal, is of such wavelength that the photoconductor is notphotosensitive thereto. In the case of cadmium sulphide, the projectionlight has a wavelength above 520 nanometers. In this mode, theinformation supplied by the imaging light is displayed in real time bythe viewing light, This is accomplished because the regions where thephotoconductor is of lower impedance due to the incidence of light, thevoltage drop is substantially taken across the liquid crystal, and thiscauses dynamic scattering in that region. In the regions where theimaging light does not occur, the principle part of the voltage drop isacross the photoconductor, so that the voltage drop across the nematicliquid crystal is not sufficient to cause scattering. Therefore, inthose areas the liquid crystal remains transparent.

In order to achieve real time imaging, the use of the combined do and acfield is preferred to the ordinary dc field provided by the dc source60. The normal decay time of the nematic liquid crystal under theinfluence of the dc field, and without illumination, may not besufficient to provide the real time display. Therefore, the superimposedac wave quickly erases the image in areas where the imaging light hasbeen removed.

In general, a cadmium sulphide photoconductor film of the typespecified, when used alone and when used in the mode necessary for theoperation described, would not have the sufficiently high impedance orsensitivity necessary for the described operation of either the cells 10or 40. By the use of the double layered structure described, thenecessary properties can be obtained.

A summary of some of the characteristics measured on laboratory modelsof the nematic-cholesteric device are summarized below:

Resolution 25 line pairs/mm Sensitivity I 0.] mj/cm"'(466.0

to 520.0 nm) Turn on 0.1 sec Erase Time -l sec Storage Time Many DaysContrast 52! (no half-tone reproduction) The sheet resistivity of thezinc sulphide insulator layer is greater than 10 ohms per square. Thesheet resistivity of the cadmium sulphide photoconductor layer in thedark state is greater than 10 ohms per square. The dark resistivity ofthe liquid crystal layer is greater than 10 ohms per square. Thecomplete cell has a light to dark current ratio of about to l. Thecomplete cell has a conductance of about 10 microamperes per squarecentimeter' at 40 V DC in the dark state.

The manner of use of the cells 10 and 40 is described with respect toFIGS. 3 through 6. Referring to FIG. 3, the cell 78 could be either ofthe cells 10 or 40, equiped with the electric application meansdescribed with respective FIGS. 1 and 2. The cell 78 is positionedbetween a source of visible light 80 which serves as a projection lightand screen 82.

A source of visible imaging light 84 provides visible radiation forphotoactivating the photoconductor layer 18 inside the-cell78, thephotoconductor layer in this instance-being on the left side of the cellfacing the sources 80 and 84. By this arrangement, images formed insidethe liquid crystal layer of the cell may be portrayed on the screen 82.As is typical, the imaging light from the source 84 is of a shorterwavelength than the display light from the source 80. If the nematicliquid crystal layer is transparent to imaging light, the cell 78 can beturned around so that the photoconductor layer is on the right side of.the cell 78 facing the screen 82 with the source 84 to the left of thecell thus being arranged to expose the photoconductor material layer bytransmission of light from source 84 through the liquid crystal layer tothe photoconductor material layer.

FIG. 4 shows the cell 86 arranged between a projection light 88 and ascreen 90. This is similar to FIG. 3 except that a partial mirror 92 isused in the optical path between the imaging source 98 and the cell 86,and in the optical path between the projection of display light 88 andthe screen 90. In this instance, the

photoconductor layer inside the cell is disposed on the side of the cellclosest to the screen with the liquid crystal layer being disposed onthe side of the cell closest to the source 88. The partial mirror 92, asindicated, has the property of transmitting light from the source 88 tothe screen 90 and reflecting light from the imaging source 94 to thecell 86. A half silvered mirror may be used for this purpose. In thealternative, which is preferred, a dichroic mirror is used for thepartial mirror 92 in which case most of the light from the source 88reaching the mirror is transmitted therethrough toward the screen 90 andmost of the light from the source 94 is reflected by the mirror towardthe cell 86. The cell 86 can be either cell 10 or cell 40.

In FIG. 5, the cell 96 is disposed in front of a cathode ray tube 90 anda lens 100. Image light from the fluorescent screen of the cathode raytube is focused onto the photoconductor material layers, of the cell 100to write an image on the photoconductor material layer. Display light,provided by ambient light or by a display light source, is reflectedfrom the cell 96 to display the resultant image written into the nematicliquid crystal layer. In FIG. 6, the cell 102 is attached to the frontpanel of the cathode ray tube 104 by a panel 106 of optical fibers whichconvey imaging light from the fluorescent display screen of the cathoderay tube to the cells photoconductor layer.

The cell 96 and the cell 102, with their electric fields, can be eitherthe type described with respect to FIG. 1, or the type described withrespect to FIG. 2. With either of these cells, the imaging light isprovided by the cathode ray tube, which provides an image in accordancewith the cathode ray tube display driven by the cathode ray tube driveinformation, identified in FIGS. 5 and 6 as the projection circuit. Thecells of FIGS. 5 and 6 have a reflected display, so that display lightis provided from the face of the cell, as contrasted to the displaylight of FIGS. 3 and 4 which was projected through the cells.

Good image quality is a high contrast ratio in the projected image. Thisimplies high light scatteringin the liquid crystal film next to theactivated regions of the photoconductor, and zero or or low lightscattering in the unactivated region s. With a single layer of CdS it isnot possible to get as good an image quality and high sensitivity inconjunction with a liquid crystal layer. The reason for this is that theCdS film by itself is not of 7 high enough dark impedance to effectivelymatch the impedance of efficient scattering liquid crystal materialQWiththe double layer structure consisting of the ZnS insulator film and theCdS photoconductor film, constructed in the manner disclosed, the darkimpedance is raised without reducing the light-to-dark current ratiowhich determines the sensitivity of the device. Thus the benefits to beaccrued are improved device performance consisting of improved contrastratio in the projected image, and improved sensitivity to the input(write) light signal.

This invention having been described in its preferred embodient, it isclear that it is susceptible to numerous modifications and embodimentswithin the ability of one skilled in the art and without the exercise ofthe inventive faculty. Accordingly, the scope of this invention isdefined by the scope of the following claims.

What is claimed is:

l. A liquid crystal device having a photoactive portion and having aplurality of layers defining the photoactive portion of said liquidcrystal device, said layers comprising:

a first electrically-conductive layer and a secondelectrically-conductive layer, said electrically-conductive layers beingconnectable for application of an electric field between saidelectrically-conductive layers;

a layer of liquid crystal which is operable in a scattering modepositioned between said electrically-conductive layers;

a layer of photoconductive material positioned between saidelectrically-conductive layers, the improvement comprising:

a layer of high electrical impedance located between saidelectrically-conductive layers so that, upon application of an electricfield between said electrically-conductive layers, the voltage divisionof the electric field between said high impedance layer, said liquidcrystal layer, and said photoconductive layer is dependent upon thestate of photoactivation of said photoconductive layer.

2. The device of claim 1 wherein said layer of high electrical impedancehas an impedance in excess of ohms per square.

3. The device of claim 1 wherein said layer of liquid crystal is a layerof nematic liquid crystal operable in the dynamic scattering mode.

4. The device of claim 1 wherein said layer of liquid crystal is a layerof cholesteric-containing liquid crystal operable in the emulsionstorage scattering mode.

5. The device of claim 4 wherein said liquid crystal is a mixture ofnematic and cholesteric liquid crystal materials and is operable in theemulsion storage scattering mode.

6. The device of claim 1 wherein said layer of high electrical impedanceis a layer of zinc sulfide.

7. The device of claim 6 wherein said layer of high electrical impedancehas an impedance in excess of 10 ohms per square.

8. The device of claim 1 wherein said layer of high electrical impedancehas a higher electrical impedance than the dark impedance of saidphotoconductor layer.

9. The device of claim 8 wherein said layer of high electrical impedanceis a layer of zinc sulfide.

10. The device of claim 1 wherein said layer of high electricalimpedance has an impedance greater than 10 ohms per square, said layerof liquid crystal has a dark impedance greater than 10 ohms per square,and said layer of photoconductive material has a dark impedance greaterthan 10 ohms per square.

1 l. The device of claim 10 wherein said layer of high electricalimpedance is a layer of zinc sulfide.

12. The device of claim 10 wherein said photoconductor layer is a layerof cadmium sulfide.

13. The device of claim 12 wherein said layer of high electricalimpedance is a layer of zinc sulfide.

14. The device of claim 13 wherein said electricallyconductive layersare cover plates, one of which is substantially transparent to light ofa wavelength to which said photoconductor layer is sensitive so thatsaid cover plates, said electrically-conductive layers, and the layerstherebetween form said cell.

l The device of claim 14 further includes means

2. The device of claim 1 wherein said layer of high electrical impedancehas an impedance in excess of 1012 ohms per square.
 3. The device ofclaim 1 wherein said layer of liquid crystal is a layer of nematicliquid crystal operable in the dynamic scattering mode.
 4. The device ofclaim 1 wherein said layer of liquid crystal is a layer ofcholesteric-containing liquid crystal operable in the emulsion storagescattering mode.
 5. The device of claim 4 wherein said liquid crystal isa mixture of nematic and cholesteric liquid crystal materials and isoperable in the emulsion storage scattering mode.
 6. The device of claim1 wherein said layer of high electrical impedance is a layer of zincsulfide.
 7. The device of claim 6 wherein said layer of high electricalimpedance has an impedance in excess of 1012 ohms per square.
 8. Thedevice of claim 1 wherein said layer of high electrical impedance has ahigher electrical impedance than the dark impedance of saidphotoconductor layer.
 9. The device of claim 8 wherein said layer ofhigh electrical impedance is a layer of zinc sulfide.
 10. The device ofclaim 1 wherein said layer of high electrical impedance has an impedancegreater than 1015 ohms per square, said layer of liquid crystal has adark impedance greater than 1012 ohms per square, and said layer ofphotoconductive material has a dark impedance greater than 1012 ohms persquare.
 11. The device of claim 10 wherein said layer of high electricalimpedance is a layer of zinc sulfide.
 12. The device of claim 10 whereinsaid photoconductor layer is a layer of cadmium sulfide.
 13. The deviceof claim 12 wherein said layer of high electrical impedance is a layerof zinc sulfide.
 14. The device of claim 13 wherein saidelectrically-conductive layers are cover plates, one of which issubstantially transparent to light of a wavelength to which saidphotoconductor layer is sensitive so that said cover plates, saidelectrically-conductive layers, and the layers therebetween form saidcell.
 15. The device of claim 14 further includes means connected tosaid first and second electrically-conductive layers for applying anelectric field between said electrically-conductive layers.
 16. Thedevice of claim 15 wherein said means is a source of direct current. 17.The device of claim 15 wherein said means is a source of alternatingcurrent.
 18. The device of claim 15 wherein said means includes sourcesof both direct and alternating current.